fix(uart): HardwareSerial begin() causes Infinite Loop #2785

baftii · 2025-08-10T19:35:32Z

I found an issue while using HardwareSerial. Sometimes, when calling begin() function, the ClockPrescaler field inside huart->Init contains a garbage value. This wrong value is then passed to HAL_UART_Init(), and later used in UART_SetConfig() → UART_DIV_SAMPLING16() macro. When this happens, the MCU triggers an interrupt and get stuck in an infinite loop.

I didn't see this problem with every HardwareSerial instance. Although I couldn't exactly find what it depends on probably it is happening when the HardwareSerial object is a member of another class, and begin() is called from that class’s constructor.

I am using PlatformIO, but I replaced framework by Arduino Core STM32 version with the latest one from GitHub. I encountered this issue while using STM32H750VBT chip.

To solve this issue I added some functions and code blocks in uart.c and uart.h files.

Added clockPrescaler setter in uart_init() function for devices which support clockPrescaler for UART. However, while setting I needed value of connected Peripharel clock and clockPrescaler finder function.
Therefore, Added a function to calculate the correct ClockPrescaler based on the UART’s PCLK frequency.
Added a helper function to get the PCLK frequency. I inspected all the datasheets while making this function but I am newbie so that this function still needs more review and testing for different STM32 series.

Tried to make the solution generic. Changes passed from CI tool but review gonna be good.

Testing

I tested on STM32H750VBT6, and it worked.

…ction added. Signed-off-by: Bartu Özcan <[email protected]>

Signed-off-by: Bartu Özcan <[email protected]>

fpistm

Hi @baftii
Thanks for this PR.
I've made a first round review with some proposal.
I will review the getPCLK deeply later.

fpistm · 2025-08-18T12:40:06Z

libraries/SrcWrapper/src/stm32/uart.c

+  // Default Value
+  uint32_t clock_prescaler = UART_PRESCALER_DIV1;
+
+  uint32_t pclk = uart_getPCLK(huart);
+  clock_prescaler = calculatePresc(pclk, baudrate, huart->Init.OverSampling);
+  huart->Init.ClockPrescaler = clock_prescaler;


I suggest to simplify by calling uart_getPCLK inside the calculatePresc, so simply pass the huart, then no more need the oversampling.

Suggested change

// Default Value

uint32_t clock_prescaler = UART_PRESCALER_DIV1;

uint32_t pclk = uart_getPCLK(huart);

clock_prescaler = calculatePresc(pclk, baudrate, huart->Init.OverSampling);

huart->Init.ClockPrescaler = clock_prescaler;

huart->Init.ClockPrescaler = uart_compute_prescaler(huart, baudrate);

fpistm · 2025-08-18T12:41:36Z

libraries/SrcWrapper/inc/uart.h

+#if defined(UART_PRESCALER_DIV1)
+uint32_t calculatePresc(uint32_t pclk, uint32_t baudrate, uint32_t oversampling);
+#endif
+
+uint32_t uart_getPCLK(UART_HandleTypeDef *huart);
+


I suggest to:

change the naming to be homogenous with other functions.

simply give the huart as arguments

avoid add uart_get_clock_source_freq when no prescaler needed as it is only used by it.

Suggested change

#if defined(UART_PRESCALER_DIV1)

uint32_t calculatePresc(uint32_t pclk, uint32_t baudrate, uint32_t oversampling);

#endif

uint32_t uart_getPCLK(UART_HandleTypeDef *huart);

#if defined(UART_PRESCALER_DIV1)

uint32_t uart_compute_prescaler(UART_HandleTypeDef * huart, uint32_t baudrate)

uint32_t uart_get_clock_source_freq(UART_HandleTypeDef *huart);

#endif

I kept uart_get_clock_source_freq outside of the guard on purpose. Although the function is used only for the prescaler configuration, the UART's source clock frequency is not directly related to the prescaler, so I thought someone can want to use it for development or experimental usages in works that nonrelated with prescaler. However, if you think keeping it within the guard would be more beneficial, I have no problem with that.

fpistm · 2025-08-18T12:44:58Z

libraries/SrcWrapper/src/stm32/uart.c

+  * @retval uint32_t clock prescaler
+  */
+#if defined(UART_PRESCALER_DIV1)
+uint32_t calculatePresc(uint32_t pclk, uint32_t baudrate, uint32_t oversampling)


Suggested change

uint32_t calculatePresc(uint32_t pclk, uint32_t baudrate, uint32_t oversampling)

uint32_t uart_compute_prescaler(UART_HandleTypeDef * huart, uint32_t baudrate)

fpistm · 2025-08-18T12:52:12Z

libraries/SrcWrapper/src/stm32/uart.c

+  static const uint16_t presc_div[12] = {1, 2, 4, 6, 8, 10, 12, 16, 32, 64, 128, 256};
+
+  uint32_t condition = 0;
+  if (oversampling == UART_OVERSAMPLING_16) {


Suggested change

if (oversampling == UART_OVERSAMPLING_16) {

if (huart->Init.OverSampling== UART_OVERSAMPLING_16) {

fpistm · 2025-08-18T12:52:21Z

libraries/SrcWrapper/src/stm32/uart.c

+  for (uint32_t idx = 0; idx < 8; idx++) {
+    uint32_t uartclk = pclk / presc_div[idx];
+    uint32_t brr = 0;
+    if (oversampling == UART_OVERSAMPLING_16) {


Suggested change

if (oversampling == UART_OVERSAMPLING_16) {

if (huart->Init.OverSampling== UART_OVERSAMPLING_16) {

fpistm · 2025-08-18T12:55:38Z

libraries/SrcWrapper/src/stm32/uart.c

+    condition = 8U;
+  }
+
+  for (uint32_t idx = 0; idx < 8; idx++) {


why it is not 12 instead of 8?

Suggested change

for (uint32_t idx = 0; idx < 8; idx++) {

for (uint32_t idx = 0; idx < 12; idx++) {

Its my bad. Firstly I wrote presc_div until 16 and after that I added values between 16 and 256. However, I forgot to change in here.

fpistm · 2025-08-18T12:56:37Z

libraries/SrcWrapper/src/stm32/uart.c

+
+/**
+  * @brief  Function called to set the uart clock prescaler
+  * @param  pclk : supplied clock rate to related uart


To be reviewed accordingly to other change.

fpistm · 2025-08-18T12:56:59Z

libraries/SrcWrapper/src/stm32/uart.c

+  * @param  huart : uart handle structure
+  * @retval uint32_t clock source frequency
+  */
+uint32_t uart_getPCLK(UART_HandleTypeDef *huart)


Suggested change

uint32_t uart_getPCLK(UART_HandleTypeDef *huart)

uint32_t uart_get_clock_source_freq(UART_HandleTypeDef *huart)

fpistm · 2025-08-18T12:58:02Z

libraries/SrcWrapper/src/stm32/uart.c

+  }
+  return UART_PRESCALER_DIV1;
+}
+#endif


like the uart_getPCLK is only used to compute the prescaler, I would add his guard after it.

Suggested change

#endif

fpistm · 2025-08-18T12:58:10Z

libraries/SrcWrapper/src/stm32/uart.c

+
+  return 0;
+}
+


Suggested change

#endif /* UART_PRESCALER_DIV1 */

baftii · 2025-08-18T14:07:11Z

Hi, I looked at your proposals for the first review and commented on a few. However, I'm a bit new to PR, so I don't know if I need to do anything right now. Do I need to batch commit your proposed changes or call a resolve conversation for proposals? Or is that all handled by you?

fpistm · 2025-08-18T14:10:18Z

Hi, I looked at your proposals for the first review and commented on a few. However, I'm a bit new to PR, so I don't know if I need to do anything right now. Do I need to batch commit your proposed changes or call a resolve conversation for proposals? Or is that all handled by you?

Simply update your code, commit and push on your branch then it will be updated here.

Signed-off-by: baftii <[email protected]>

baftii added 2 commits August 6, 2025 13:02

fix(uart): uart clock prescaler configuration and uartPCLK getter fun…

48b8e12

…ction added. Signed-off-by: Bartu Özcan <[email protected]>

fix(uart): fixed wrong values with definitions

7a1c8ed

Signed-off-by: Bartu Özcan <[email protected]>

fpistm reviewed Aug 18, 2025

View reviewed changes

fix(uart): fixes according to review.

28d4f18

Signed-off-by: baftii <[email protected]>

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fix(uart): HardwareSerial begin() causes Infinite Loop #2785

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	uint32_t calculatePresc(uint32_t pclk, uint32_t baudrate, uint32_t oversampling)
	uint32_t uart_compute_prescaler(UART_HandleTypeDef * huart, uint32_t baudrate)

	if (oversampling == UART_OVERSAMPLING_16) {
	if (huart->Init.OverSampling== UART_OVERSAMPLING_16) {

	for (uint32_t idx = 0; idx < 8; idx++) {
	for (uint32_t idx = 0; idx < 12; idx++) {

	uint32_t uart_getPCLK(UART_HandleTypeDef *huart)
	uint32_t uart_get_clock_source_freq(UART_HandleTypeDef *huart)

fix(uart): HardwareSerial begin() causes Infinite Loop #2785

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